1. Field of the Invention
This invention relates to an improved Liquid Crystal Display (LCD) device which is used in products ranging from watch displays to flat panel colour TV screens
The Liquid Crystal Device industry is currently a multi-billion dollar industry. In this industry, the products range from simple watch displays to flat panel colour TV screens. The device described in the present invention has advantages over the conventional LCD devices in that it has a wide and symmetrical viewing angle, no reversal of the contrast ratio in any direction, and also results in a simplification of the fabrication process. Accordingly, the device of the present invention will be very useful for various applications in liquid crystal industry.
When molecular crystals are heated to their melting point they usually change into the liquid phase. The periodic structure of the lattice as well as the orientational ordering of the molecules are destroyed simultaneously. However if the constituent molecules have a pronounced anisotropy of shape, such as a rod or a disc, the melting of the lattice may precede the disappearance of the orientational ordering. One, then, has an intermediate phase composed of molecules which are more or less parallel to each other, but at the same time exhibiting a certain degree of fluidity. The molecules can slide over on one another while still preserving their parallelism. The fluid is therefore anisotropic, turbid and, like a crystal, shows optical birefringence and dielectric anisotropy. At a higher temperature there is orientational melting and the anisotropic fluid transforms into the ordinary isotropic clear liquid. Such intermediate phases, which occur as a result of heating or cooling, are referred to as thermotropic liquid crystals.
Thermotropic liquid crystals can be classified broadly into two types, (i) those composed of rod-shaped molecules (called xe2x80x9ccalamiticxe2x80x9d liquid crystals) which are known from the end of 19th century and form the majority of the currently known liquid crystals and (ii) those composed of disc-shaped molecules (called xe2x80x9cdiscoticxe2x80x9d liquid crystals) which have been developed recently.
2. Description of the Related Art
Discotic liquid crystals which were discovered by us as recently as 1977 represent a new class of thermotropic liquid crystal. In this context reference may be made to the publication of S. Chandrasekhar, B. K. Sadashiva and K. A. Suresh, Pramana, 9, 471-480 (1977). In this case the discs are stacked one on top of the other to form columns, the different columns constituting a two-dimensional lattice. The basic columnar structure is shown in FIG. 1 of the drawing accompanying this specification. A number of modifications of the above said basic structure have been identified. Some discotic compounds exhibit a nematic (ND) phase as well. It is a fluid phase consisting of an orientationally ordered arrangement of discs, but with no long range translational order, somewhat like a pile of coins as shown in FIG. 2. However, unlike the usual nematic calamitic, the nematic discotic is optically negative. The preferred orientation of the axis of the disc is termed as the director as shown in FIG. 2.
The advent of discotic liquid crystals triggered off a spate of activity in this field and well over a thousand discotic compounds have been reported to date. A few discotic compounds are exemplified below: Hexaalkonoyloxy benzenes, hexaalkoxy triphenylenes, bis-(4-n-decylbenzoyl)methanato copper (II), hexa-n-alkanoates of truxene and octasubstituted phthalocyanines.
The potential uses of such materials are as quasi-one-dimensional conductors, photoconducting systems, ferroelectrics, light emitting diodes, photovoltaic solar cells, optical data storage devices and hybrid computer chips for molecular electronics.
Display devices based on calamitic liquid crystals are well known. A widely used device is the twisted nematic (TN) display device. In a twisted nematic display device two transparent glass plates are coated on their inner surfaces with a thin layer of transparent electrically conducting material, such as indium tin oxide, and further with a thin layer of a polyimide. The method of unidirectionally rubbing the said substrates with cotton or rayon or nylon fabric is widely used to achieve a macroscopic orientation of the liquid crystal director. The two glass plates are held apart at a distance of approximately 6-10 xcexcm by means of spacers to form a cell, with the rubbing directions of the polyimide layers orthogonal to each other. The gap between the substrates of the cell is filled with a calamitic nematic liquid crystal. Owing to the boundary conditions the nematic liquid crystal will become oriented parallel to the rubbing direction of each glass plate and consequently the director will undergo a twist of 90xc2x0 over the nematic layer distance. Polarizer sheets are attached to the outer surfaces of the glass plates with the axis of vibration (polarizing axis) of each sheet parallel to the rubbing direction of the plate to which it is attached. Unpolarized light is transformed into linearly polarized light by the polarizer fixed on the entrance side of the cell and emerges on the exit side with the polarization axis rotated through 90xc2x0. The emergent light will be transmitted by the second polarizer. Thus in this configuration, the so called normally white mode, the display appears bright in the unactivated state. A white mode with enhanced viewing angle can be achieved by setting the polarizers with their polarizing axes perpendicular to the rubbing directions. The application of an electric field normal to the layer orients the liquid crystal molecules (of positive dielectric anisotropy, xcex94xcex5 greater than 0) with their long axes along the layer normal. In this activated state the polarization axis of light is not rotated by the liquid crystalline medium and the display appears black. Orientation of one polarizer parallel and the second polarizer perpendicular to the rubbing direction results in a black appearance in the unactivated state and a bright appearance in the activated state. This so called black mode is useful for automobile dash board applications.
The major disadvantage of the above type of device is that when it is viewed obliquely, the viewing angle characteristic is poor, resulting in a loss of contrast, and even contrast inversion at certain azimuthal angles. See FIG. 3, which is taken from the publication of Y. Toko, T. Sugiyama, K. Katoh, Y. Imura and S. Kobayashi, J. Appl. Phys., 74, 2071-75 (1993) showing a typical polar plot of the contrast ratio (CR) for a conventional TN device.
Another widely used device is the supertwisted nematic (STN) device. The construction of such a device is similar to that of a TN device explained above except that the twist angle of the director is between 180xc2x0 and 270xc2x0, instead of 90xc2x0. The higher twist angle is achieved by incorporating a suitable quantity of a chiral compound as a dopant in the nematic material before it is filled into the cell. However this device does not lead to any improvement in the viewing angle characteristic.
Both the TN and STN devices suffer from the additional disadvantage that for multiplexed displays there is a large difference in the pixel capacitance between the ON and OFF states, which gives rise to the problem of cross talk between pixels.
The viewing angle profiles, the symmetry and the angle dependence of the intensity contrast ratio between the ON and OFF states of any display device are important criteria for determining the quality of performance of the device. Several attempts have been made to enhance the performance of such devices. These attempts are directed mainly to improve the viewing angle characteristics using different techniques such as dividing each pixel into sub-pixels, adding retardation films, or by applying an electric field parallel to the substrate plane. The noteworthy point in all these attempts is the fact that the liquid crystalline material used is of the nematic calamitic type. No attempts have been made to date employing nematic discotic material for fabricating a display device, or to using same as the electrically switchable material.
For example, regarding the modification based on applying an electric field parallel to the plane of the substrates reference may be made to the publications of G. Baur, R. Kiefer, H. Klausmann and F. Windscheid, Liquid Crystal Today, 5, 13-14 (1995); M. Oh-e, M. Yoneya and K. Kondo, J. Appl. Phys., 82, 528-535 (1997); S. H. Lee, H. Y. Kim, I. C. Park, B. G. Rho, J. S. Park, H. S. Park and C. H. Lee, Appl. Phys. Lett., 71, 2851-2853, (1997). In this method the authors have improved the viewing angle characteristics of liquid crystal devices by employing in-plane electrodes on only one of the substrates and by avoiding the assymmetry of the director profile.
In the publications of H. Mori, Jpn. J. Appl. Phys., 36, 1068-1072 (1997); H, Mori, Yoji Itoh, Yosuke Nishiura, Taku Nakamura, Yukio Shinagawa, Jpn. J. Appl. Phys., 36, 143-147 (1997) an optical compensator with negative birefringence was introduced to reduce the amount of light leakage in the dark state.
In the publications of K. H. Yang, Jpn. J. Appl. Phys., 31, L1603-1605 (1992) and J. Chen, P. J. Bos, D. R. Bryant, D. L. Johnson, S. H. Jamal, J. R. Kelly, SID 95 Digest, 865-868 (1995) the authors have employed multiple domains of the liquid crystals in which the orientation of the director was different in each of the domains (pixels).
The devices fabricated as mentioned above do not improve the viewing angle characteristics of the device satisfactorily. Further they also involve additional steps in the fabrication process.
In the publications of Y. Toko, T. Sugiyama, K. Katoh, Y. Iimura and S. Kobayashi, SID 93 Digest, 622-625 (1993); J. Appl. Phys., 74, 2071-75 (1993) a simpler process for producing LCD having improved viewing angle characteristics has been disclosed. In this process polymer films are coated on the transparent conducting substrates, but no rubbing is done. The non-rubbed polymer film is optically and structurally isotropic and the director is parallel to the surfaces of the substrates but randomly oriented in the plane of the substrate in the OFF state. In the ON state the director is normal to the surfaces of the substrates. This so called amorphous TN device gives an improved viewing angle characteristic, free from contrast inversion. See FIG. 4, which is taken from the publication of Toko et. al. referred to above. It is to be noted that the liquid crystal material used in this device is again of the nematic calamitic type and not of the nematic discotic type.
Other prior art devices are embodied in U.S. Pat. No. 5,833,685 to Mazaki et al., and U.S. Pat. No. 5,730,900 to Kawata et al. Kawata uses a discotic compound for encapsulating and orienting the LC molecules of the switchable material. The discotic material does not play a part in the electrical switching process. Mazaki teaches an optical compensating film of fixed orientation made of discotic material located between the liquid crystal cell and the polarizing plates. This discotic film serves as an optical compensator and increases the viewing angle relative to the viewing angle of the uncompensated display. However, the compensating film made of discotic material of negative optical anisotropy plays no part in the switching process. Accordingly, neither of these references contemplate the advantages to be achieved by the novel structure discussed below.
Recognising the importance of improving the viewing angle characteristics of liquid crystal devices we undertook intensive research work in this direction. Our sustained research work resulted in our finding that if nematic discotic materials are employed in liquid crystal devices as the electrically switchable material, the viewing angle characteristics of such devices can be further enhanced.
Therefore the main object of the present invention is to provide a liquid crystal device having improved viewing angle characteristics employing nematic discotic material.
Another object of this invention is to provide a liquid crystal device having no reversal of the contrast ratio in any direction.
Yet another object of the present invention is to provide a liquid crystal device with reduced difference in the pixel capacitance between the ON and the OFF states resulting in lowering of the cross talks substantially in the case of a multiplexed display device.
Still another object of the present invention is to provide a liquid crystal device employing a simple fabrication process.
To meet above objects, the invention, for the first time, provides a liquid crystal device comprising nematic discotic material as the electrically switchable material.